1. Field of the Invention
The present invention relates to a servo control for a disk drive data storage system. More particularly, the present invention relates to the formation of servo information in a hard disk drive and a method for its servo control.
2. Description of the Related Art
A hard disk drive has been extensively used as one of the most preferred and efficient auxiliary memory means for computer systems. The entire operation of a hard disk drive is controlled by a microprocessor which is connected to a programmable read only memory. The read only memory stores a set of necessary basic operating software and servo control algorithms.
A static random access memory is also provided. A read/write head is horizontally moved in and out on the surface of a recording medium such as a disk upon reading and writing data from and to the disk. A voice coil motor is coupled to the head, serving as an actuator for driving the head in a horizontal direction on the disk during the read/write operation and a voice coil motor driver is coupled to the voice coil motor to control its operation. A spindle motor is coupled to the disk for driving a high speed rotation of the disk on its central axis.
A digital to analog converter receives a series of digital control signals from microprocessor and converts the digital control signals to analog signals for input to the voice coil motor driver. A motor driver is coupled to the spindle motor to control its rotation under the control of the microprocessor.
An amplifier is connected to the head to effect an amplification of a read data signal retrieved by the head as well as a write data signal to be recorded on the disk so as to provide a proper amplitude level of signal.
An interface controller serves as an interface to communicate various data with any peripheral circuits inclusive of a host computer as well as the microprocessor.
A read/write channel circuit is coupled to the amplifier and the interface controller and further to the microprocessor through a gate array and an analog to digital converter so as to receive the write data from the interface controller under the control of the microprocessor, and encodes the received write data according to the predetermined coding logic prior to delivery to the amplifier.
Furthermore, the read/write channel circuit effects a digital conversion of the analog read signal from the amplifier to provide encoded read data. The analog to digital converter is connected to the read/write channel circuit to receive an analog servo reading signal and to convert the received signal into a digital signal for input to the microprocessor.
The gate array circuit is also connected to the read/write channel circuit to receive the encoded read data signal to generate the various servo information such as a gray code in a servo region of the disk from the received encoded read data signal.
In the aforementioned hard disk drive, a precise servo control is needed during the read/write operation to relocate the head to a desired target track and to have the head accurately follow its center course along the target track. Servo information necessary for the servo control is usually recorded on the disk by a servo writer during a manufacturing stage of a hard disk drive, wherein two kinds of recording methods are generally utilized by manufactures, that is, a so-called "dedicated" servo method for recording the servo information using a single entire surface of a plurality of hard disks and an "embedded" servo method for recording the servo information as well as the write data on the same disk surface. The dedicated servo method is mainly utilized for a hard disk drive having at least four disks in a stack while the embedded servo method is utilized for a hard disk drive consisting of a smaller number of disks than the dedicated servo method.
In the embedded servo method, the servo sectors for servo information and the data sectors are alternately arranged. The respective servo sector consists of an automatic gain control region, a servo address mark region, and index region, a gray code region, a servo burst region, and a PAD region. The automatic gain control region also functions as a read/write recovery region and serves to constantly maintain the time necessary for changing the head from a data write mode to a servo information read mode as well as a magnitude of position signal read out of the head throughout the entire disk area. The servo address mark region is for recording a reference pattern for generation of the various servo timings and the index region is for providing a unit revolution information of the disk. The gray code region includes the respective numbers of a servo sector, a head and a cylinder, and the servo burst and PAD regions are used to control on-track positioning of the head.
As the aforementioned servo information often leads to a reduction of the data storage capacity for a user of the hard disk, it is accordingly appreciated that the reduction of such servo information enables one to increase the data storage capacity permitted to the user. According to the current state of the art, it is noted that the servo information area requires at least 10 to 15% of the entire disk area. As a result, the smaller the area of the servo region used, the larger the data storage area obtained for a user in a hard disk drive, which may result in the reduction of the production costs
The Yu patent, U.S. Pat. No. 5,589,998, entitled Cylinder Address Storage In Multiple Servo Sectors OF A Track, discloses a cylinder address storage arrangement for multiple servo sectors of a recording track in which partial cylinder addresses are stored on each servo sector of each track to reduce the servo data.
The Blagaila et al. patent, U.S. Pat. No. 5,526,202, entitled Disk Drive Embedded Servo System Having A Servo Field With An Asymmetrical Position Subfield, discloses a disk drive arrangement in which the servo fields are of different sizes so as to reduce the servo overhead.
The Shinohara et al. patent, U.S. Pat. No. 5,311,379, entitled Method And System For Positioning Head On Target Cylinder Of Disk, discloses an arrangement in which the servo signal used to drive the head position signal includes at least first and second servo signal components on even cylinders of the magnetic disk and third and fourth servo signal components on odd cylinders of the magnetic disk.
The following additional patents each disclose features in common with the present invention but are not as pertinent as the patents noted above: U.S. Pat. No. 5,268,803 to Sugita et al, entitled Disc Memory Apparatus Utilizing Detection Of High Accuracy Address Data In Short Servo Sectors For High SpeedAccessing, U.S. Pat. No. 5,073,834 to Best et al., entitled Disk File Or Tape Drive With High Servo Sample Rate Embedding Additional Servo Sectors Within Individual Data Sectors, U.S. Pat. No. 5,339,207 to Moon et al., entitled Servo System For Providing with the same data storage capacity or an increase of the data storage capacity with a like disk size or the same cost.
Thus, so far, various improvements to reduce a total size of the servo information have been proposed in the state of the art by achieving more effective formation of the servo information sector in a hard disk drive. Generally, the reducible portions of the servo information may be the AGC region, the gray code region and the servo burst region. Unfortunately however, the AGC region could not be further reduced below a given physical size because it mainly depends upon performance of the head and the read/write channel. Furthermore, the servo burst region also could not be further reduced below a given size because it is to be limited by the filter characteristic of a servo signal portion of the read channel and its reproducing circuit portion. In addition, with the headerless servo writing pattern, a servo sector number and a head number as well as a track address, namely, a cylinder number are recorded together in the respective gray code region, so that it often leads to enlargement of the gray code region, thereby resulting in failure of extension of a total size of data regions in a hard disk drive.
Moreover, enlargement of the gray code region not only causes the decrease of the data region, but also makes a disk more susceptible to its defects, which disk is often forced to be completely replaced by a defect-free new disk due to the defects found or to require a kind of special defect management program for compensation for any defects. Therefore, the above drawbacks mostly results in an increase of the production cost for a hard disk drive or the deterioration of the reliability and stability in operation of a disk. Increased Recording Density And Improved Operation Of The AGC Circuitry, U.S. Pat. No. 5,579,186 to Yamamoto etal., entitled Magnetic Disk Unit And Method For Controlling The Same, U.S. Pat. No. 5,561,566 to Kigami et al., entitled Information Recording Disk And Drive System Using No Sector ID To Increase Data Storage Capacity, U.S. Pat. No. 5,523,903 to Hetzler et al., entitled Sector Architecture For Fixed Block Disk Drive, U.S. Pat. No. 5,581,418 to Hasebe, entitled Magnetic Disk Drive Unit Capable Of Determining Data Region Position Of Data Region That Does Not Include Position Identification Data, and U.S. Pat. No. 5,596,460 to Greenberg et al, entitled System And Method for Encoding A Servo Address.